Wind turbines extract energy from wind with their rotor blades. While an airflow, i.e. the wind, passes over a rotor blade a lift force is created, which lift force is a result of a pressure difference between a suction side and a pressure side. From these lift forces there results a rotational movement of the rotor which comprises those rotor blades, which rotational movement is then used to generate power. That power output is then fed for instance into a utility grid. Depending on the wind speed the extractable energy changes. Modern up-to-date wind turbines operate relatively closely to the Betz limit (i.e. the theoretical maximum portion of rotational energy which can be extracted from a given wind at a given speed) under normal operating conditions. Generally, it is the aim to optimize wind turbine operation such that the most of extractable energy under given boundary conditions (such as wind speed, load, lifetime of the wind turbine and/or of parts thereof, noise, wear etc.) can be supplied.
State-of-the-art wind turbines feature a pitch system with which each rotor blade can be rotated around its longitudinal axis. With pitching the blade its pitch angle and as a consequence its so-called angle of attack can be changed and adjusted to achieve optimum operation for given conditions, for instance rotational speed of the rotor, tip speed of the rotor blades and wind speed.
The lift force and energy extracted from the wind increases with an increase of the angle of attack until a critical angle of attack is reached. Thereby, the angle of attack is the angle between the chord line of the rotor blade and the direction of the inflowing air arriving on the rotor blade.
A stall will occur if the angle of attack is further increased beyond the critical angle of attack, which is called the “stall angle” throughout this description.
When the rotor blade stalls, the airflow over the rotor blade separates from the rotor blade and a turbulent airflow dominates on the suction side of the rotor blade. The stall may cause an abrupt loss of lift on the rotor blade and thus an abrupt loss of extracted energy from the wind. In addition, unwanted loads are exerted on the wind turbine and unwanted, often considerably loud sounds are generated.
The occurrence of such stall (i.e. the stall angle) is also dependent on the condition of the blade, namely on its wearing condition and its soiling condition. For a typical wind turbine profile the surface condition can be influenced by dents, cracks and eyes (all of which are caused by ageing and thus wearing) and by accumulation of ice (icing) and/or of dust and dirt (i.e. by soiling).
The soiling and/or ice accumulation on a rotor blade can make the occurrence of a stall of the rotor blade more likely. That means a stall on a soiled and/or iced rotor blade can occur at an angle of attack αt which a clean rotor blade will still operate smoothly. The same applies analogously to worn rotor blades. Under fixed conditions a worn, iced or soiled rotor blade thus stalls at a lower angle of attack than a proper, i.e. unworn, iceless and clean, rotor blade. The problem with this is that the wearing, icing and soiling conditions of a rotor blade is not completely predictable or determinable and can vary. For instance, after a rain shower dirt might have been washed off the rotor blade and a stall would occur at a relatively high angle of attack. The determination of soil accumulation on the blade with sensors is however complex, expensive and not a matured enough technology. Consequently, wind turbines cannot be operated at their very optimum performance:
To avoid an unwanted stall the wind turbine is normally run conservatively within a high security margin of about 1° to 2° of the angle of attack to always compensate for a possibly soiled, iced or worn blade and the stall conditions due to those factors. This implies a power loss of about 1% to 2% due to the security margin. Operation of the wind turbine with such a pitch strategy normally avoids stalls and their unwanted effects. However, this pitch strategy also leaves potential for improved energy extraction as the wind turbine is not run at its full potential i.e. at its optimum angle of attack given the circumstances available for each rotor blade.
FIG. 1 shows a wind turbine 1 according to the state of the art: It comprises a number of rotor blades 3 of a rotor 9 which rotor 9 is connected to a nacelle 7. The nacelle 7 is positioned on top of a tower 5 which tower 5 is firmly connected to the ground 11. The ground 11 may be a monopile 11 installed offshore but also an onshore ground 11.
When the rotor 9 is rotated due to the incoming wind, its rotational movement is transferred into the nacelle 7 via a drivetrain (not shown). In the nacelle 7, there are positioned a gearbox and a generator (not shown) which latter generates electric power from the rotational movement. Alternatively, a generator can also be positioned in a transition zone of the wind turbine between the rotor and the nacelle; the generator then being realized as a direct drive generator, which implies that no gearbox is necessary.
FIG. 2 shows one rotor blade 3 of the wind turbine 1. It extends from a blade root 15 to a blade tip 13 and has a leading edge 17 and a trailing edge 19. The wind during wind turbine operation hits the rotor blade 3 at the leading edge 17 and flows along the profile of the rotor blade in the direction of the trailing edge 19.
FIG. 3 shows an airfoil of the rotor blade 3 at the cross-section of FIG. 2 along a section line III-III. The line 21 going through both the leading edge 17 and the trailing edge 19 is the so-called chord line 21, which represents the main (i.e. maximum) cross-sectional extension of the rotor blade 3. Wind hits the rotor blade 3 at a certain direction WD at an angle α to the chord line 21. This angle α is generally referred to as the angle of attack α. The angle of attack α can be varied indirectly by varying the pitch angle of the rotor blade 3, i.e. by pitching the rotor blade 3 around its longitudinal axis, i.e. the axis that runs from the blade root 15 to the blade tip 13. Therefore, the angle of attack α is dependent (amongst other factors such as wind speed, and rotational speed of the rotor) on the pitch angle: by rotating the rotor blade 3 in the counter-clockwise direction in FIG. 3, the value of the angle of attack α is reduced, whereas by pitching the rotor blade 3 in the clockwise direction in FIG. 3, the value of the angle of attack α is increased.
The rotor blade 3 can be divided into a suction side Su and into a pressure side Pr, the suction side Su being on the one side of the chord line 21 and the pressure side Pr being on the other side of the chord line 21. Which side of the chord line 21 is the suction side Su und which side is the pressure side Pr depends on the angle of attack α. The side of the rotor blade 3 to which the angle of attack α is directed can be considered the pressure side Pr whereas the side of the rotor blade 3 from which the angle of attack α faces away can be considered the suction side Su. A lifting force F is generated due to the influence of the wind, i.e. due to suction forces on the suction side Su and to pressure forces on the pressure side Pr. Thereby, the lifting force F arises from a pressure gradient (or pressure difference between the suction side Su und the pressure side Pr according to Bernoulli's principle. The lifting force F forces the rotor blade 3 to force the rotor 9 to rotate around its rotational axis.
There is, however, a limit to that system, namely a critical angle of attack. This can be observed in FIG. 4: Over the angle of attack α the lift coefficient C1 is depicted in the diagramme. Two lines L1, L2 refer to a behaviour of a soiled rotor blade (L1) and of a clean rotor blade (L2) both being rotor blades of the same age and thus wearing conditions and both not being covered by ice. Thus, the two rotor blades essentially only differ with respect to their soiling conditions. It can be observed that the lower curve L1 of the soiled rotor blade reaches its maximum value of lift coefficient C11 at a lower angle of attack αMax1 than the upper lift curve L2 of the clean rotor blade. Rather the clean rotor blade's maximum value of lift coefficient C12 is higher than C11 and is reached at a higher angle of attack αMax2. In both curves L1, L2, it can be observed that shortly after having reached the angles of attack αMax1/αMax2 the lift coefficient C1 drops down quite rapidly. That means that the stall angle is reached very early after the angles of attack αMax1/αMax2 at which the maximum lift coefficient C11, C12 can be measured. In other words—the soiled rotor blade stalls at a lower angle of attack than the clean one.
Certainly, a very soiled rotor blade, a very worn rotor blade or a very iced rotor blade or indeed a combination of a soiled and iced, a soiled and worn and/or an iced and worn rotor blade can even lead to a situation in which the security gap indicated above may not be enough. Therefore, it is highly desirable to have a possibility of finding out more precisely the state of a rotor blade of a wind turbine with respect to the three mentioned factors, i.e. soiling, icing and wearing.